In U.S. Pat. No. 3,645,922, Weiss e.a. describes a composite ion exchange resin of particulate acid and base ion exchange materials dispersed in a homogeneous matrix of a water-insoluble, crosslinked, hydrophilic polymer such poly (vinyl alcohol). This composite resin is particularly suitable for use in the demineralization of salt solutions by the "Sirotherm" process. In this Australian process for the partial demineralization of brackish water, a mixed bed of weak acid and weak base resins is used to remove dissolved salts from the brackish water. The weak acid and weak base resins are so chosen that the electrolyte adsorption characteristics are thermally reversible. For example, at 25.degree.C the mixed bed will adsorb salt from the water, but when the temperature is raised to 85.degree.C, the equilibrium shown in Eq. 1 will be reversed to release the adsorbed salt. ##EQU1## Thus a resin bed saturated with salt at ambient temperature, can be thermally regenerated by washing with hot water and recycled.
Extensive work has been done on this process. Initial pilot plant studies have been reported by Battaerd e.a., Desalination, 12, 217-237 (1973). It has been recognized that since a proton must be transferred from one weakly ionized particle to another, small particles are essential for acceptable absorption and regeneration kinetics. Thus Battaerd e.a. U.S. Pat. Nos. 3,619,394 and 3,716,481 describe quasispherical triallylamine polymers having a mean diameter of 0.5-10 microns for use in the Sirotherm process.
Since the thermally reversible resin is the key component in this process, improved resin structures are highly desirable.
Another approach to composite ion exchange resin particles is described in Hatch U.S. Pat. Nos. 3,041,292, 3,205,184 and 3,332,890 where an essentially linear polymer is formed within the interstices of a conventional ion exchange resin by absorbing and polymerizing a suitable monomer in situ. Thus ar-vinylbenzyltrimethylammonium hydroxide was polymerized within a sulfonated styrene-divinylbenzene resin to give a composite "snake-cage" resin effective in absorbing neutral salts from aqueous solution. These composite resins are particularly effective in separating solutes by the ion retardation process of Hatch U.S. Pat. No. 3,078,140. Extension of this concept to the preparation of weak acid-weak base snake-cage resins gave thermally reversible resins with fast kinetics but unacceptably low salt capacities.